CVE-2025-21664 (GCVE-0-2025-21664)

Vulnerability from cvelistv5 – Published: 2025-01-21 12:18 – Updated: 2025-11-03 20:58
VLAI?
Title
dm thin: make get_first_thin use rcu-safe list first function
Summary
In the Linux kernel, the following vulnerability has been resolved: dm thin: make get_first_thin use rcu-safe list first function The documentation in rculist.h explains the absence of list_empty_rcu() and cautions programmers against relying on a list_empty() -> list_first() sequence in RCU safe code. This is because each of these functions performs its own READ_ONCE() of the list head. This can lead to a situation where the list_empty() sees a valid list entry, but the subsequent list_first() sees a different view of list head state after a modification. In the case of dm-thin, this author had a production box crash from a GP fault in the process_deferred_bios path. This function saw a valid list head in get_first_thin() but when it subsequently dereferenced that and turned it into a thin_c, it got the inside of the struct pool, since the list was now empty and referring to itself. The kernel on which this occurred printed both a warning about a refcount_t being saturated, and a UBSAN error for an out-of-bounds cpuid access in the queued spinlock, prior to the fault itself. When the resulting kdump was examined, it was possible to see another thread patiently waiting in thin_dtr's synchronize_rcu. The thin_dtr call managed to pull the thin_c out of the active thins list (and have it be the last entry in the active_thins list) at just the wrong moment which lead to this crash. Fortunately, the fix here is straight forward. Switch get_first_thin() function to use list_first_or_null_rcu() which performs just a single READ_ONCE() and returns NULL if the list is already empty. This was run against the devicemapper test suite's thin-provisioning suites for delete and suspend and no regressions were observed.
Severity ?
No CVSS data available.
Assigner
References
Impacted products
Vendor Product Version
Linux Linux Affected: b10ebd34cccae1b431caf1be54919aede2be7cbe , < ec037fe8c0d0f6140e3d8a49c7b29cb5582160b8 (git)
Affected: b10ebd34cccae1b431caf1be54919aede2be7cbe , < cd30a3960433ec2db94b3689752fa3c5df44d649 (git)
Affected: b10ebd34cccae1b431caf1be54919aede2be7cbe , < 802666a40c71a23542c43a3f87e3a2d0f4e8fe45 (git)
Affected: b10ebd34cccae1b431caf1be54919aede2be7cbe , < 12771050b6d059eea096993bf2001da9da9fddff (git)
Affected: b10ebd34cccae1b431caf1be54919aede2be7cbe , < 6b305e98de0d225ccebfb225730a9f560d28ecb0 (git)
Affected: b10ebd34cccae1b431caf1be54919aede2be7cbe , < cbd0d5ecfa390ac29c5380200147d09c381b2ac6 (git)
Affected: b10ebd34cccae1b431caf1be54919aede2be7cbe , < 80f130bfad1dab93b95683fc39b87235682b8f72 (git)
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    Linux Linux Affected: 3.15
Unaffected: 0 , < 3.15 (semver)
Unaffected: 5.4.290 , ≤ 5.4.* (semver)
Unaffected: 5.10.234 , ≤ 5.10.* (semver)
Unaffected: 5.15.177 , ≤ 5.15.* (semver)
Unaffected: 6.1.125 , ≤ 6.1.* (semver)
Unaffected: 6.6.72 , ≤ 6.6.* (semver)
Unaffected: 6.12.10 , ≤ 6.12.* (semver)
Unaffected: 6.13 , ≤ * (original_commit_for_fix)
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\ndm thin: make get_first_thin use rcu-safe list first function\n\nThe documentation in rculist.h explains the absence of list_empty_rcu()\nand cautions programmers against relying on a list_empty() -\u003e\nlist_first() sequence in RCU safe code.  This is because each of these\nfunctions performs its own READ_ONCE() of the list head.  This can lead\nto a situation where the list_empty() sees a valid list entry, but the\nsubsequent list_first() sees a different view of list head state after a\nmodification.\n\nIn the case of dm-thin, this author had a production box crash from a GP\nfault in the process_deferred_bios path.  This function saw a valid list\nhead in get_first_thin() but when it subsequently dereferenced that and\nturned it into a thin_c, it got the inside of the struct pool, since the\nlist was now empty and referring to itself.  The kernel on which this\noccurred printed both a warning about a refcount_t being saturated, and\na UBSAN error for an out-of-bounds cpuid access in the queued spinlock,\nprior to the fault itself.  When the resulting kdump was examined, it\nwas possible to see another thread patiently waiting in thin_dtr\u0027s\nsynchronize_rcu.\n\nThe thin_dtr call managed to pull the thin_c out of the active thins\nlist (and have it be the last entry in the active_thins list) at just\nthe wrong moment which lead to this crash.\n\nFortunately, the fix here is straight forward.  Switch get_first_thin()\nfunction to use list_first_or_null_rcu() which performs just a single\nREAD_ONCE() and returns NULL if the list is already empty.\n\nThis was run against the devicemapper test suite\u0027s thin-provisioning\nsuites for delete and suspend and no regressions were observed."
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